External resonance-type semiconductor lasers include one that includes therein an optical reflector, wavelength filter, and a semiconductor optical component having an optical amplification function and formed on a semiconductor substrate, and uses a cleaved facet of the semiconductor optical component as a reflector for configuring a resonator. In order to afford another function, such as an optical modulation function, to the semiconductor laser, an optical modulator should be separately mounted thereon in some form. There are roughly two techniques for mounting an external optical modulator. In one configuration, as shown in FIG. 18, an optical modulator 211, such as LN modulator or semiconductor optical modulator, is separately prepared, and optically coupled to a laser resonator 220, which includes a semiconductor optical amplifier 203, a lens 206, a wavelength filter 202, and an optical reflector 201, via an optical component such as a lens in a module, thereby allowing the module to have an optical modulation function as a whole. The optical modulator 211 is coupled to an optical fiber in the module via an optical isolator 207. In the other configuration, as shown in FIG. 19, an optical modulator module 214 is separately prepared, and the optical output of a semiconductor laser module 213 is guided to the optical modulator module 214 via an optical fiber, to thereby provide an optical modulation function on a board.
However, the configuration wherein the module is provided with the optical modulation function, as shown in FIG. 18, increases the number of components in the module to incur a cost increase. In addition, the complicated configuration in the module increases the man-hour of the assembly and decreases the throughput. Further, there also arises the problem of an increased size of the module. The configuration shown in FIG. 19, wherein the optical modulation function is provided on the board, necessitates provision of the optical modulator module and the semiconductor laser module separately, thereby incurring problems such as a cost increase and a larger occupied area of the modules on the mounting board. For solving those problems, it is desired that an optical reflector mechanism corresponding to a cleaved facet be formed on the semiconductor substrate, and the optical modulator is monolithically integrated on the same substrate.
PCT Patent Publication JP-2003-508927A describes the configuration wherein an optical modulator is monolithically integrated on a semiconductor substrate in an external resonance-type semiconductor laser. In JP-2003-508927A, a partial reflector is configured on a substrate including a semiconductor component having an amplifying function of light, and the optical modulator, is monolithically integrated thereon. In this publication, the partial reflectors proposed therein include:                1) etched facet;        2) loop mirror; and        3) distributed Bragg reflector (DBR).In particular, if it is intended to replace the cleaved facet (facet obtained by cleavage) by the reflector, the etched facet shown in item 1) offers a promising prospect.        
A variety of reports have been provided heretofore as to the integrated structure using the etched facet. Examples of the structure include one using an etched trench as a filter, and another using the etched facet as a reflector. Among those structures, the structure focusing on “raising the reflectivity” has been reported in a number of proposals. For example, Patent Publication JP-1998-125989A describes a multilayer reflector mirror including low-refractive-index/high-refractive-index layers, and that “more than two layers” employed achieve a reflectivity as high as around 92%. In JP-1998-125989A, it is recited that this configuration realizes a semiconductor laser having a lower threshold current. It is also proposed that the semiconductor laser having a lower threshold current be integrated with another functional region.
Contrary to JP-1998-125989A, a structure focusing on “decreasing the reflectivity” is also reported. Examples of the report include JP-1999-14842A, wherein an isolation trench is formed on a semiconductor substrate by etching, and a semiconductor layer having a refractive index equivalent to the refractive index of the semiconductor substrate is embedded in the isolation trench, thereby suppressing the reflectivity at the interface which originally configures the facet part. JP-1995-20359A recites a configuration wherein an isolation trench is formed, an optical spot incident onto the trench has a spot size as larger as 5 μm, and the facet is bent by around 10° whereby the attenuation amount of the reflection is 50 dB or above, with the reflectivity of an ordinary vertical facet being a unit.
A structure focusing on “decreasing the optical coupling coefficient” as to the integrated structure using the etched facet is also reported. Examples of the report include JP-2004-200697A, wherein an isolation trench is referred to as inner window, and a larger distance for the isolation trench is employed, to thereby achieve a drastic decrease in the optical coupling coefficient between the regions isolated from one another by the isolation trench and having different functions. JP-2004-221321A describes a structure wherein two functional regions are simply isolated from one another.
It is desired in a semiconductor laser, wherein an optical modulator is monolithically integrated via an isolation trench on a semiconductor substrate and an optical reflection mechanism corresponding to the cleaved facet is formed on the semiconductor substrate, that the optical reflection mechanism have a suitable reflectivity not higher than around 30% (corresponding to that of facet) and not lower than several percent for achieving a higher-laser-efficiency operation and that the optical connection mechanism have as high an optical coupling coefficient as possible, such as 50% or above, in order for achieving a sufficient level of the optical output power of the optical modulator. However, JP-2003-508927A only describes that an etched facet is used, and thus it is not clear whether or not the above suitable reflectivity and high optical efficiency can be obtained. JP-1998-125989A pursues a reflectivity of 90% or above, which is too high to achieve the above suitable reflectivity and high coupling coefficient, and thus the targeted reflectivity and high optical coupling coefficient cannot be obtained.
In JP-1999-14842A and JP-1995-20359A, reduction of the reflectivity is focused and the interface part (facet part) has a reflectivity of substantially zero. Accordingly, it is almost impossible to achieve the above suitable optical reflectivity and high optical coupling coefficient. Although an example of butt-joint between an optical coupler/splitter using a quartz-based PLC and a semiconductor laser is also reported, even in such a case, coating with a refractive-index matching gel or forming an AR film for antireflection on the facet is used in order for suppressing the reflection between those devices as much as possible, and thus the technological orientation thereof is directed to suppressing the refection as much as possible. In JP-2004-200697A, the optical coupling coefficient is lowered for optical isolation, and thus unable to obtain the suitable optical reflectivity and higher optical coupling coefficient, especially unable to obtain the latter. JP-2004-221321A proposes only the structure for isolation between the two functional regions, and thus there is a lower possibility of achieving the above suitable reflectivity and higher optical coupling coefficient.
As described heretofore, the semiconductor laser of conventional techniques including an optical reflector, a wavelength filter, and a semiconductor optical component formed on a semiconductor substrate and using the facet of the semiconductor component as an optical reflector configuring the resonator fails to achieve the structure that provides a facet optical reflector in the semiconductor optical component having the suitable reflectivity, which is not higher than around 30% (corresponding to that of the cleaved facet) and not lower than several percent, and provides an optical coupling coefficient not lower than 50% and as high as possible.